There is an on-vehicle apparatus control system in which control of an on-vehicle apparatus is performed, such as locking and unlocking of a door, and starting of an engine, on the basis of radio signals which are transmitted and received between an on-vehicle control device mounted on a vehicle and a portable machine carried by a user. Communication methods between the on-vehicle control device and the portable machine are roughly classified into three methods such as a polling method, a passive entry method, and a keyless entry method.
In the polling method, the on-vehicle control device transmits a response request signal at a predetermined cycle regardless of a position of the portable machine. If the response request signal is received, the portable machine returns a response signal to the on-vehicle control device. In the passive entry method, when a user comes close to or comes into contact with a door knob, a passive request switch is turned on, and the on-vehicle control device transmits a response request signal to the portable machine. If the response request signal is received, the portable machine returns a response signal to the on-vehicle control device. In the keyless entry method, a response request signal is not transmitted from the on-vehicle control device to the portable machine, and, when the user operates the portable machine, a remote control signal is transmitted to the on-vehicle control device from the portable machine. In any case, if the response signal or the remote control signal is received from the portable machine, the on-vehicle control device performs collation between ID codes included in the signal. If the collation is successful, the on-vehicle control device locks or unlocks a door of the vehicle, or starts an engine thereof.
The portable machine operates with power of a battery built thereinto. In order to increase a service life of the battery of the portable machine, for example, in JP-A-2008-127777, a transmission output level of a signal from a portable machine is switched during passive entry and during keyless entry. Specifically, during keyless entry in which an operation switch of the portable machine is operated, a remote control signal is transmitted in a first transmission output level from the portable machine. During passive entry in which a passive request switch is operated, a response signal is transmitted in a second transmission output level lower than the first transmission output level from the portable machine. In a case where the response signal transmitted from the portable machine cannot be received by an on-vehicle apparatus, a response signal is transmitted again in the first transmission output level from the portable machine.
Meanwhile, in a case of the polling method or the passive entry method, for example, an illegal communication act may be performed in which the portable machine which is far away disguises itself as being in close proximity to a vehicle as a result of a relay relaying a response request signal transmitted from the on-vehicle control device and the portable machine receiving the response request signal. The illegal communication act using the relay is called relay attack. A malicious third party who is not an owner of a vehicle may commit a crime such as theft of the vehicle by unlocking a door of the vehicle or starting an engine through the relay attack.
Therefore, regarding crime prevention countermeasures against the relay attack, for example, in JP-A-2012-051421, an attenuation rate of a signal transmitted from a portable machine is switched during passive entry and during keyless entry. Specifically, during keyless entry in which an unlock switch of the portable machine is operated, an RF transmission unit of the portable machine attenuates a door unlocking signal to a low attenuation rate, and transmits the door unlocking signal to an on-vehicle apparatus. During passive entry in which a passive request switch (touch sensor) is operated, the RF transmission unit of the portable machine attenuates a response signal (an identification signal including an ID code) to a high attenuation rate, and transmits the response signal to the on-vehicle apparatus.
If an output level of a signal transmitted from the portable machine is simply reduced, or an attenuation rate is merely increased as relay attack countermeasures, the relay attack can be suppressed, but there is a concern that the transmitted signal (electric wave) may be canceled out by external noise, and thus may not be received normally by the on-vehicle control device.
In a case where an output level or an attenuation rate of a signal transmitted from the portable machine is switched as relay attack countermeasures, a circuit for the changing is required to be provided in the portable machine. However, in a case where such a circuit is provided, a circuit configuration or signal processing becomes complex, and this is contrary to demands for miniaturization of the portable machine.